1. Field of the Invention
The present invention relates to a spindle motor which is used to rotate an optical disc such as a compact disc (CD), a CD-ROM or the like.
2. Description of the Prior Art
FIG. 1 is a side view showing a conventional type of spindle motor as described above. In FIG. 1, a spindle motor 1 comprises a stator portion 2 which is fixedly disposed, and a rotor portion 3 which is supported so as to be rotatable relatively to the stator portion 2.
The stator portion 2 includes a stator board 2a, a bearing holder 2b secured to the stator board 2a, a wound coil 2c which is fixedly disposed so as to surround the bearing holder 2b in the peripheral direction, and a bearing 2d which is held in the bearing holder 2b by press-fitting or the like. The bearing 2d is formed as an oil-impregnation plain (sliding) bearing by metal sintering, for example, and lubricating oil is impregnated in pores 2e (FIG. 2) which occur in the sintering process.
Further, the rotor portion 3 includes a rotating shaft 3a which is rotatably supported by a bearing 2d, a cylindrical rotor yoke 3b which is opened at the lower end thereof and fixed to the rotating shaft 3a so as to surround the wound coil 2c from the outside, a rotor magnet 3c disposed on the inner surface of the rotor yoke 3b, a substantially disc-shaped turn table 3d which is fixed in the neighborhood of the upper end of the rotating shaft 2. In this case, the lower portion of the rotating shaft 3a is supported by a thrust pad 4 in the bearing 2d, whereby the rotating shaft 3a is prevented from falling down.
According to the spindle motor 1 thus constructed, an optical disc (not shown) is disposed on the turn table 3d, and the magnetic field which is generated in the wound coil 2c by suitably supplying current to the wound coil 2c interacts with the magnetic field due to the rotor magnet 3c and the rotor yoke 3b of the rotor portion 3, whereby the rotor portion 3 is rotated. At this time, when following the rotation of the rotor portion 3, the rotating shaft 3a is rotated in the bearing 2d, the lubricating oil impregnated in the bearing 2d seeps into a slight gap between the rotating shaft 3a and the bearing 2d to form oil membrane between the rotating shaft 3a and the bearing 2d, so that the rotating shaft 3a is smoothly rotatable in the bearing 2d.
However, in the spindle motor 1 thus constructed, upon following the rotation of the rotating shaft 3a, the lubricating oil 2f seeping into the gap between the rotating shaft 3a and the bearing 2d may be moved along the rotor yoke 3b by a centrifugal force occurring due to the rotation of the rotating shaft 3a as indicated by an arrow A of FIG. 2, so that the lubricating oil is finally scattered to the outside. Therefore, the amount of the lubricating oil in the gap between the rotating shaft 3a and the bearing 2d is reduced, and thus an oil membrane which enables the smooth rotational motion of the rotating shaft 3a cannot be formed. In this case, there occurs a problem that the rotating shaft 3a is baked and bonded to the bearing 2d, so that the rotor portion 3 is not normally rotated and thus the lifetime of the spindle motor 1 is reduced.